a) Field of the Invention
The present invention relates to a fuel-injection system for an internal-combustion engine.
b) Background of the Related Art
Known, in the framework of compression-ignition engines for motor vehicles, are injection systems (the so-called common-rail systems) consisting of by a plurality of electro-injectors supplied by a common storage volume of fuel under pressure.
In particular, operation of said injection systems envisages that a low-pressure priming pump will draw the fuel from a tank and will make it available to a high-pressure pump. The high-pressure pump compresses the fuel up to the pressure of injection and makes it available to a common storage volume, which supplies the electro-injectors. A pressure-regulation system enables the desired pressure to be maintained within the storage volume.
The pump may be of the type with one or more pumping elements with reciprocating motion that generate a single delivery line, which feeds the common storage volume. Each pumping element performs each time a suction stroke and a compression or delivery stroke.
One of the functions of the common storage volume is that of dampening the pressure oscillations caused by the delivery of fuel from the high-pressure pump to the storage volume and by the extraction of fuel caused by opening of the electro-injectors.
In detail, the electro-injectors are supplied by the common storage volume and inject the fuel nebulized at high pressure into each of the combustion chambers of the respective engine cylinders. With reference to the current state of the art, there is felt the need to reduce the volume of the common storage volume in order to meet more satisfactorily current standards on pollutant emission.
In greater detail, in the engine-starting stage, the high-pressure pump is driven by the engine of the motor vehicle, and hence there occurs a transient period, during which the common storage volume is at a pressure lower than the steady-state pressure, and the electro-injectors take in fuel to start the engine itself. The duration of this transient increases as the size of the common storage volume increases. The injection of fuel by the electro-injectors during this transient causes non-optimal operation of the internal-combustion engine and in particular increases the emission of pollutant substances.
Furthermore, the reduction in volume of the common storage volume would enable reduced overall dimensions and a more convenient installation in the internal-combustion engine.
However, the reduction in volume of the storage volume could entail drawbacks in the use of the injection system during steady running conditions. In particular, opening of the electro-injectors causes a pressure drop in the common storage volume. Said pressure drops are dampened by the storage volume in a way that is the more effective the greater the volume of the storage volume itself. Consequently, in the case where the volume of the storage volume were insufficient to dampen the aforesaid pressure drops, operation of the electro-injectors would be faulty, and the pollutant emissions of the internal-combustion engine would increase.